With the popularization of portable audio players, noise reduction systems that provide a satisfactory reproduced sound field space by reducing noise of an external environment for headphones or earphones for portable audio players or reducing external noise for listeners have begun to spread.
For example, Patent Literature 1 discloses a technique of a noise reduction system capable of generating a noise cancellation signal of an opposite phase in which a sound pressure of noise becomes minimum at ears of a listener using a noise signal collected through a microphone that collects ambient noise and cancelling noise.